The problem of environmental pollution of estuaries is viewed in the wider perspective, including the role of mixing, as a process common to estuaries and other systems and regulated by spatial variations. Increasing mobilization of heavy-metals and other pollutants to the estuarine environment, imparting hazards to Man in the long run, is a cause for concern and provides the reasons and bases for environmental monitoring. Already total lessons have been learnt from catastrophic outbreaks of mercury poisoning in Japan.

During the winter of 1974–75, both tar and aromatic hydrocarbons were sampled in the Mediterranean Sea. A detailed report of the tar findings were given in a previous paper (Morris et al., 1975). Aromatic hydrocarbon concentrations at a depth of 1 m averaged 167 ng/1 (expressed as phenanthrene equivalents). This is only about 60% as great as the average aromatic hydrocarbon concentration found in the Baltic Sea, but is 5 times greater than what was found in the north-west Atlantic—including the Sargasso Sea (Zsolnay, 1977).

No relationship was found between tar and aromatic hydrocarbon concentrations. This would indicate that the concentration of the latter is dependent upon other factors—such as the presence of other organic compounds that are capable of keeping the hydrophobic hydrocarbons in the water column by ‘solubilization.’

The biological and geological importance of mangroves in coastal environs is well recognized. These halophytes contribute leaf-fall to marine food-webs, act as sediment stabilizers, and serve as a habitat for quite numerous organisms (Odum, 1971; Carlton, 1974; Walsh, 1974). They are consequently of considerable ecological and environmental significance. Attempts to revegetate coastal areas that have become denuded of valuable mangrove cover are also important environmentally and have involved two approaches: planting propagules (seedlings) or transplanting older, established plants (Teas, 1977).

The basis of the physico-social pattern in Ethiopia, indicated above, may be summarized as:

(a) The main features of the country are formed by volcanic activity;

(b) There exist a lot of fragmented highlands with a great range of altitude, climate, and vegetation;

(c) The content of organic matter in the soils is very low on the eroded lands, greatly limiting their ability to retain water;

(d) There exist a wide variety of cultural patterns and numerous spoken languages;

(e) There are very severe erosion hazards and continuous deforestation activities;

(f) There is a custom of uncontrolled grazing, and livestock are not limited according to the carrying capacity of the land;

(g) There are constraints in the distribution and quality of water that is available for domestic supplies, livestock, and crops;

(h) From ancient times there has been no substantial improvement in farming tools and implements;

(i) There is little government investment in development work in general;

(j) The importance of a conservation programme for a wide range of natural resources is not felt/realized by the government;

(k) There is no emphasis on erosion control and forestry management—even by the regional development projects of the minimum and maximum package projects financed by the World Bank and other bilateral international cooperations;

(l) There are big constraints in matters of education and agricultural extension services; and

(m) The mountainous terrain and deep gorges have made communication difficult and expensive.

As a result of the above, the overall development network does not at present provide an adequate access to support a productive and continuous agriculture.

In urban areas, continuing fragmentation of natural habitat, disturbance, and increasing isolation of individual ‘habitat islands’, has brought on almost general reduction in species richness. Sensitive species are being replaced by aggressive synanthropic ones. Continued loss seems inevitable, but reducing the rate of that loss is a worthwhile conservation goal. It is imperative that the numerous and varied habitats within each urban area be considered as interrelated and not as separate units. Island biogeography models supply the means for unifying the disparate elements in urban ecological studies and can provide a useful strategy for conservation.

It is difficult to define colonization, extinction, and equilibrium, in disturbed, often transient, urban habitats. Pseudo- and successional turnover dominate. The occupation of each ‘habitat island’ is a function of its own geography and of its position relative to other ‘islands’. Each habitat may serve as isolated ‘island’, ‘stepping-stone’, or ‘corridor’, depending on its spatial relationships with other ‘habitat islands’ and with the nature of each organism present.

‘Habitat islands’ in small cities appear to function like large or near oceanic islands, while those in large cities seem to respond like small or distant oceanic islands. In all cases, the analogy with land-bridge islands is appropriate. Continuing urbanization is leading to reduced ‘habitat island’ size, and increasing the isolation of units from one another and from the surrounding rural ‘reservoir’.

Whilst major new industrial development sometimes poses a threat to scenic values and ecologically important habitats, it is also evident that industry has contributed significantly to the present-day biological resources of the British Isles. In particular, several former mineral-extraction sites have been maintained and developed specifically for the security and management of wildlife, and now constitute important habitats for the protection and survival of less-common members of the British flora. Several locations have even been successfully defended on biological grounds against proposals to reutilize them for disposing of industrial wastes and domestic refuse.